It is known to use oxidation agents such as, for example, permanganate and chromates for the purpose of inhibiting the corrosion of aluminum in alkaline solutions. The degree of effectiveness of such oxidation agents is, however, low, so that relatively large quantities must be used. Today, chromates can practically no longer be used for safety and ecological reasons involving the work environment and effluent treatment.
The use of water glass (sodium silicate) as an inhibitor for aluminum in alkaline solutions is also known. Good results are achieved with water glass if it is used in appropriately large quantities. It has, however, been shown that the necessarily large addition of water glass to alkaline solutions often leads to unpleasant accompanying phenomena. For example, incrustations and coatings occur on the parts which have been treated with the solutions, especially if these parts are subsequently treated with acid in order to remove excess alkali. Such incrustations and coatings are not attacked by conventional de-scaling solutions. Their removal is in practice only possible by means of hydrofluoric acid and is thus complicated and not without its problems.
It has already been proposed to use 1-amino alkane 1,1-diphosphonic acid as an inhibitor for aluminum in alkaline solutions. Good results can thereby be achieved but, from the standpoint of effluent treatment, it is, however, desirable to use compounds whose phosphorus and nitrogen content is as low as possible.